1. Field of the Invention
This invention relates to apparati and methods for desmearing the holes of multi-layered printed wiring boards by flowing a plasma gas therethrough. More particularly, this invention relates to apparati and methods for compensating for the saturation of the plasma gas as it becomes saturated during the desmearing of the printed circuit boards.
2. Description of the Prior Art
Plasma etching or desmearing is a technique utilizing active gases which are applied to organic surfaces such as epoxy, and more specifically in the hole of a printed wiring board, resulting in a chemical reaction which cleans the hole of unwanted smear. The reactant products are volatile, usually inert, and are pumped away through the vacuum system. A better appreciation of plasma technology as applied to state of the art printed wiring boards will be better understood with an explanation of smear, and the wet processes used to remove such smear.
Smear in a plated-through hole in a multi-layer wiring board is defined as the presence of an insulative or foreign material or combination of both over the conductive inner circuit, which, if not removed, causes poor electrical interconnect after the electrodeposition of copper in the hole of a multi-layer printed wiring board (hereinafter MPWB).
Smear is a natural result of printed wiring board drilling operations. A few possible causes of drill smear include improper lamination cycle (causing resin curing problems) resulting in "soft spots" on the board, substandard resin characteristics, substandard drills in the form of dullness, incorrect geometry, speed, feed, incorrect back-up material, and board hole design. Technological advances in drilling and lamination have reduced the quantity of smear, but it is still virtually impossible to produce smear-free holes economically in production quantities.
The most commonly used procedure to remove smear by MPWB manufacturers is called the etchback process. This process begins with using either sulfuric acid or chromic acid and hydrofluoric acid or ammonium bifluoride/hydrochloric acid. At times, various combinations are used. After the acid treatment, the panels are rinsed in distilled or city water, neutralizing solution, and a final rinse. Thereafter they are either blown dry or heat dried. Modifications of these procedures for cleaning the holes depend on each manufacturer's requirements, but they are essentially a wet process.
Concentrated acids and their byproducts (corrosive fumes) are very undesirable since they pose serious health hazards and unsafe work environments. It is important to observe that the etchback process, while removing smear, can create its own set of problems. For example, after etchback, if the MPWBs are not properly rinsed and neutralized, they can leave salt deposits which can be transferred into the plating baths, this contaminating the solutions, or the salts can be trapped inside, between the hole wall and the copper barrel, thus migrating laterally to yield poor insulation resistance and/or high resistance shorts. These shorts may not be detected visually and electrically at final inspection but can degrade the MPWBs in the field. This reduces product life and reliability. To resolve this condition, all MPWBs have to be thoroughly rinsed. If possible, rinsing should take place under pressure or agitation or a combination of both. The large amount of rinse water employed must be adequately treated prior to disposing of it as waste. The etchback process also leaves the hole wall ragged, and secondary treatment is required to condition the hole wall prior to the electrolytic deposition of copper or other materials.
The most significant drawback in the wet process is the lack of control. This is due to the fact that the rate of etchback is a function of solution concentrations, temperature, time and other factors. When there is no steady-state system available to maintain the proper chemical concentrations, it becomes a time-based function. The time-based function, in turn, is almost impractical to calculate from one lot to the next. As a result the wet process becomes a guessing game, and an art rather than a science.
More recently it has been demonstrated that the use of plasma technology can be applied to removing smear instead of wet-acid process. "Plasma" is defined as matter in an "elevated state". Physical properties of matter in the plasma state are somewhat similar to those in the gaseous state. The flow patterns and characteristics become extremely agressive in the plasma state, where the gases are essentially in a nascent form. Plasma or these "active species" are created in a region between a pair of RF (radio frequency) electrodes, and are then directed toward the target surface. In this instance the example is directed to the holes in the MPWBs.
For desmearing with plasma of the MPWBs, various gases can be used depending upon the individual manufacturer's requirements and the type of laminate material. The gases more generally used are oxygen (O.sub.2) and a carbontetrafluoride (CF.sub.4). When a mixture of these gases passes through the RF electrodes in a vacuum, the active species formed are said to be nascent oxygen and atomic fluorine. These active species react with the polymer glass combination in the holes of the MPWB to form volatile products which are subsequently removed by exhausting.
The desmearing process using plasma gas basically requires a chamber, a vacuum pump, a source of RF energy, and metered gases. The chamber must withstand the vacuum, but more importantly, it must accommodate MPWBs so that the proper directional flow of the active species is possible, and so that there is an even flow of these species throughout the matrix of boards to achieve uniform results. This may be accomplished by placing the boards closest to the gas source and exhaust port a certain distance away. This distance is termed as a transition area. Once the chamber is designed, the vacuum pump must be selected with the proper throughput capability and compatibility with the gases used. The requirements for the source of RF energy depend upon the electrode surface area and power density requirements (Watts per square inch, not just Watts).
Several manufacturers are offering plasma desmearing systems which include a cylindrical chamber with a front loading door and either a barrel or planar electrode configuration. The MPWBs are processed either vertically or horizontally. The gas inlet is located on the top barrel wall, usually toward the front of the chamber. The exhaust valves are usually near the back wall. With this configruation, plasma is generated within the body (which acts as ground) and the electrodes. With a barrel electrode, the intensity of the field varies as it approaches the center of the chamber. Planar electrodes provide better distribution of the field, so long as they are paired (power and ground,) since the gases within the plasma state reduce to ash the undesirable substrate material.
The flow characteristics of the gases are a function of the gas inlet at a relatively higher pressure to the exhaust outlet at a low pressure. Experiments have shown that the plasma gas becomes saturated as the plasma gas flows from the gas inlet to the exhaust outlet during the desmearing process. Accordingly, the MPWBs located near the gas inlet tend to be desmeared more than those MPWBs located near the exhaust outlet. Moreover, the flow of the plasma gas tends to concentrate about the peripheral edge portions of the MPWBs such that the holes located near the outer portions of the MPWBs are desmeared more than those holes located in the central portion of the MPWBs. In each instance, it is almost impossible to achieve the uniform results in the desmearing of MPWBs of each lot.
Accordingly, it is an object of this invention to provide an apparatus which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant improvement in the desmear/etchback art.
Another object of this invention is to provide an apparatus and method for desmearing which is of significant importance to desmearing the drilled holes in MPWBs.
Another object of this invention is to provide an apparatus and method for plasma desmearing of MPWBs in a controlled process with a high level of precise repeatability and at cyclic rates substantially reducing the overall cost of desmearing.
Another object of this invention is to provide an apparatus and method for spacing MPWBs within a chamber for desmearing which results in a uniform treatment of the holes within the MPWBs irrespective of whether the MPWBs are located central of the unit or at the ends thereof.
Another object of this invention is to provide an apparatus and method for mixing or blending various gases for desmearing thereby readily accommodating various types of MPWBs for desmearing.
Another object of this invention is to provide an apparatus and method for increasing the intensity of the electric fields produced by succeeding electrode pairs to compensate for the saturation of the plasma as the plasma flows from one end of the chamber to the other.
Another object of this invention is to provide an apparatus and method for varying the intensity of the electric field produced by each pair of electrodes whereby the electric field decreases from the central area of the electrode pair to the outer peripheral portions thereof thereby assuring that all of the holes located in the MPWBs are uniformly desmeared during the desmearing process.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.